The present invention relates to a capacitive sensing array device comprising an array of sense electrodes carried on a substrate and covered by a layer of dielectric material defining a sensing surface.
A device of the above described kind and intended for sensing capacitively fingerprint patterns in particular is described in U.S. Pat No. 5,325,442. The device compares a plurality of sense elements arranged in a row and column matrix array. Each sense element consists of a sense electrode connected to an associated switching device in the form of, for example, a thin film transistor (TFT) and the switching devices of the array are connected to a peripheral drive circuit via sets of row and column address conductors carried on the substrate and extending between the sense electrodes. The drain electrode of each TFT is connected to the sense electrode of the sense element. When a finger is placed over the dielectric material, the sense electrodes together with the overlying layer of dielectric material and individual fingerprint portions constitute capacitors. The row address conductors are connected to a scan circuit which applies a selection signal to each row conductor in sequence to turn on the TFTs of the sense elements of the row. Simultaneously with a selection signal a potential is applied to the column address conductors to charge the capacitors. The individual capacitances of these capacitors depend on the spacing of the fingerprint portions from the sense electrodes, as determined by the presence of a ridge of a trough of the fingerprint, and are measured by sensing the charging current flowing in the column conductors during charging of the capacitors, using current or charge sensing amplifier circuits incorporated in the drive circuit. At the end of the address period, the TFTs are turned off and a gating signal is applied to the next row conductor to turn on the TFTs of the next row of sense elements. Each row of sense elements is addressed in this manner in turn and the variation in sensed capacitances produced over the array of sense elements by a fingerprint ridge pattern provides an electronic image or representation of the fingerprint pattern. In addition to the array being formed using thin film technology, with the transistors comprising thin film transistors (TFTs) on an insulating substrate, for example of glass or plastics, the array may alternatively comprise an integrated circuit using a silicon substrate.
In a modification of one embodiment of fingerprint sensing device described in this specification, the structure includes metal grounding conductors provided on the surface of the dielectric layer and overlying the spaces between the sense electrodes, either in a grid pattern or as linear conductors, for the purpose of improving electrical contact to the finger surface.
In a separate embodiment also described, each sense element is provided with a second, electrically isolated, electrode on the surface of the dielectric layer which is of a similar size to the sensing electrode and arranged overlying the sense electrode. The second electrodes are intended in use to be contacted and grounded by ridges of a person""s fingerprint placed thereon so as to define together with their underlying sense electrodes and intervening dielectric material substantially identical, and more distinctive, capacitors at the fingerprint ridge locations.
A similar kind of structure, comprising an active matrix array with capacitive sensing electrodes but occupying a larger area and in which the pitch of the sense electrodes is increased could be used as a touch input device such as a graphics tablet, responsive to a person""s finger or a stylus.
A problem with such a capacitive sensing array device is that an electrostatic charge from the body of a person touching the device or even bringing a finger into proximity to the sensing surface can cause damage, particularly to the switching devices which are susceptible to such charges.
It is an object of the present invention to provide an improved capacitive sensing array device which is less susceptible to damage being caused in this manner.
According to the present invention there is provided a capacitive sensing array device of the kind described in the opening paragraph wherein each sense electrode is associated with a second electrode provided on the sensing surface and overlying the sense electrode, wherein conductor lines connected to a source of predetermined potential extend over the sensing surface in the regions between the sense electrodes, each second electrode being in close proximity to, but spaced from, at least one of said conductor lines extending adjacent thereto, and wherein, with regard to each second electrode and a conductor line adjacent thereto, at least one is shaped at least at one part thereof in an electric field enhancing manner so as to encourage electrical flash-over at that part at a reduced potential level.
As a result of using this structure, the risk of damage being caused to the device, and especially components such as the switching devices (e.g. transistors) associated with the sense electrodes, through electrostatic charges carried by a person when touching the device, or more especially when bringing a body part into proximity with the device, is considerably reduced. The invention involves recognition that two structural components which are described separately in different embodiments described in U.S. Pat. No. 5,325,442, namely the provision of (grounded) conductor lines and second, isolated, electrodes overlying the sense elements, can be used in combination beneficially for this purpose, and that by designing one or both of these components appropriately to define field enhancing regions the device can be afforded considerable protection from the unwanted effects of electrostatic discharges in a highly controlled and definable manner. Although the grounded conductor lines in the device of the embodiment described in the aforementioned publication could perhaps serve to dissipate charge carried by a persons finger when brought into direct physical contact therewith, there would inevitably be a substantial risk, particularly when a person""s finger is merely being brought close to the sensing surface, of such charge jumping instead in an uncontrolled, ill-defined, way through the dielectric material to an underlying sense electrode and damaging the associated switching device rather than jumping to a conductor line. The provision of a second, isolated, electrode alone would not offer protection as electrostatic discharges could either merely jump via that second electrode to the underlying components, or if the charge only jumps to the overlying electrode then a large voltage excursion will occur on the overlying electrode which is capacitively coupled to the sense electrode causing a voltage change large enough to damage the switching devices.
The invention, on the other hand, provides reliable protection from the effects of electrostatic charges by rendering such effects harmless and limiting voltage swings occurring on the sense electrodes by ensuring that at higher voltages, i.e. levels which could cause damage or destruction to components, breakdown occurs in a reliable and controlled fashion whereby any voltage excursions on the sense electrodes are reduced and kept below a threshold at which damage can be caused. At relatively low voltages, impedance to ground is very high so the normal operation of the array is unaffected.
A further, secondary, advantage of the invention is that the conductor lines and second electrodes can together occupy a substantial proportion of the area of the sensing surface and by employing appropriately hard metal materials for these parts a higher degree of scratch resistance is afforded to the exposed surface of the sensor array.
The conductor lines may extend linearly between adjacent rows or columns of sense electrodes and preferably are in the form of a grid with each sense electrode being surrounded by a grid portion.
Either the second electrode or the adjacent conductor line, or both, may be shaped at one or more adjacent regions to provide field enhancement. Such shaping may typically take the form of sharp or other pointed projections. Thus, each second electrode and/or the portions of conductor line or lines extending adjacent thereto may have one or a plurality of shaped projections. Where projections are provided on both elements they preferably face one another to define thereat a reduced spacing where charge can readily flash over. The exact position of flash-over is thus highly defined.
Although the invention is particularly beneficial in capacitive type fingerprint sensing devices of the kind described for example in U.S. Pat No. 5,325,442, it of course can be beneficially applied in other kinds of capacitive sensing array devices where similar problems with electrostatic charge can be experienced.